The Voyager probes were devised as part of the Planetary Grand Tour, a mission to learn more about the outer planets. The probes would use the gravity of Jupiter as a planet-sized slingshot to extend their reach to the further planets and eventually interstellar space. If they ever happen to encounter intelligent life, each Voyager space is carrying a time-capsule in the shape of an audio-visual disc. On the disc are photographs of life on Earth, spoken greetings and music by Mozart, Blind Willie Johnson and Chuck Berry.